1. Money - I actually got some Yen in Singapore before the Christmas holidays and brought Singapore dollars as pocket money. My uncle suggested that we exchange our foreign currencies at the airport. I exchanged only half of the SGD I brought and didn't realize that banks close for the New Year holiday and that it was hard to find a money changer in the city and that they only exchange US dollars (argh! but I saved some money because of that). So, best to exchange foreign currencies at the airport or just bring USD to be sure or better yet get some Yen beforehand.
2. SIM - I hate having to roam especially for data. I'm really scared of bill-shock so I always buy a local SIM. I always get one wherever I travel, but I was really surprised that it's hard to get one in Japan. At the airport they only rent out pocket wifis at $18/day (so expensive!). We found prepaid sim cards being sold a Lawson (convenience store), but the shop keeper said that it was a different size and they were expensive as well at Y3,500++. We didn't buy one since they could not reassure us that the sim would fit my phone. My uncle lent me one of his sim cards, but that one didn't work either. So we decided to just do all our research at my uncle's home and enjoy the whole day without internet.
3. Public wifi - there is public wifi in Tokyo, but you are basically required to register and activate your account (which requires connectivity to activate). We managed to get wifi with help from a good friend who shared access through her phone so we could activate our free account. Best to check the night before from your hotel/home and register before going to your intended location the next day. Stay offline, it's actually more fun!
Jorudan was our best friend for our train routes (aside from Google Maps), but we still had my uncle check our chosen routes to be sure.
5. Spend - my friends said that everything's expensive in Japan, I think though it depends on where you go and where you eat. We managed to save a lot since relatives took us in so you can definitely visit Japan without breaking your bank account. Here's what we generally spent on (a very practical approach IMHO):
Food - about Y1,000-1,200 is a good budget per meal (approximately PhP500/meal)
Train trips - about Y1,000-1,200 per day depending where you are going
Daiso budget - it's just Y100 per item (just about PHP36-37!!!)
Disney - bought tickets online at Y6,400 per person which also gave us the benefit of getting into the express lane for rides and we just walked in the park, skipping the long lines to buy a ticket
Akihabara - Y10,000 will already buy you quite a lot of Manga/toys; just make sure you have a list of what you want to buy to save time looking for it, bring much more of course if you are buying electronics/devices.
It's winter now in Japan, a friend said that it's better to buy winter clothes at your destination and she was right because there was a huge crazy sale going on in Japan when we were there and we got additional winter clothes at discounted prices. It gets really cold so make sure you have the appropriate clothes (and heat pad!) so you don't get sick.